High gate leakage current of silicon oxide and nitrided silicon dioxide as well as depletion effect of polysilicon gate electrodes limits the performance of conventional semiconductor oxide based gate electrodes. High performance devices for an equivalent oxide thickness (EOT) less than 1 nm require high dielectric constant (high-k) gate dielectrics and metal gate electrodes to limit the gate leakage current and provide high on-currents. High dielectric constant typically refers to a dielectric constant grater than 4.0. Materials for high-k gate dielectrics include ZrO2, HfO2, other dielectric metal oxides, alloys thereof, and their silicate alloys.
A high-k dielectric material needs to provide good electrical stability, that is, the amount of charge trapped in the high-k dielectric material needs to remain at a low level even after extended operation of a transistor. The high-k dielectric material needs to be scalable, that is, provide an acceptable level of leakage and acceptable levels of electron and hole mobility at a reduced equivalent oxide thickness (EOT), e.g., less than 1 nm. High-k dielectric materials satisfying these conditions may be advantageously employed for high performance semiconductor devices.
The electrical characteristics of semiconductor devices employing high-k dielectric materials are affected by the oxygen content of the high-k dielectric materials. For example, field effect transistors that employ a gate dielectric including a high-k dielectric material displays variations in the threshold voltage depending on the oxygen content of the high-k dielectric material. Most field effect transistors have a gate dielectric that extends over an active area and a shallow trench isolation structure that surrounds the active area. The effect of the threshold voltage variation may be dependent on the width of the field effect transistor because extra oxygen may be supplied from the shallow trench isolation structure containing silicon oxide into the high-k gate dielectric. The oxygen may laterally diffuse into the portion of the gate dielectric overlying the active area, thereby affecting the threshold voltage of the field effect transistor.
The effect of the oxygen diffusion on the threshold voltage of a field effect transistor is severer on an edge portion of the field effect transistor that is located proximate to the shallow trench isolation structure. For a field effect transistor having a narrow width, i.e., a width that is comparable with the diffusion length of oxygen from a surrounding shallow trench isolation structure, the threshold voltage shift is severe. For a field effect transistor having a wide width, i.e., a width that is far greater than the diffusion length of oxygen from a surrounding shallow trench isolation structure, the threshold voltage shift is less because the effect of the threshold voltage shift is limited to the periphery of the field effect transistor, while the center portion of the field effect transistor displays insignificant, if any, shift in the threshold voltage.
Formation of an oxygen diffusion barrier layer on the entirety of the surface of a semiconductor substrate degrades the device characteristics because the oxygen diffusion barrier layer effectively functions as a portion of a gate dielectric. Formation of an oxygen barrier layer only on the top surface of shallow trench isolation structures by deposition and lithographic patterning increases processing complexity and cost. Nitridation of shallow trench isolation structures invariably induces nitridation of exposed semiconductor surfaces, which degrades device characteristics of semiconductor devices formed on the semiconductor surfaces. If the diffusion of oxygen from shallow trench isolation structures into high-k gate dielectrics is not deterred, the semiconductor devices formed on the semiconductor substrate may have a width dependent variation in the device characteristics such as the threshold voltage of a field effect transistor.